Enhanced mechanical properties and plastic instability regulation of medium Mn steel via cyclic intercritical annealing and cold rolling

This study explores the effects of cyclic intercritical annealing and cold rolling on medium Mn steel to achieve a high-strength multilayer structure. As the treatment progresses, the microstructure transforms from micrometerscale austenite into a complex structure consisting of martensite + micrometer-scale austenite layers with nanometer-scale gamma L + alpha L layers. This transformation is characterized by reduced layer thickness and an increased proportion of nanometer-scale grains, leading to grain refinement. In the first three cycles, grain refinement, dislocation accumulation, element partition between gamma L and alpha L, and enhanced austenite stability all contribute to the improvement of the steel's strength and plasticity. After three cycles, the steel achieves a tensile strength of 1903 MPa and an elongation of 22.3 %. However, during the fourth cycle, the micrometer-scale austenite occurs recrystallization, resulting in decreased austenite stability and dislocation density, which leads to a reduction in tensile properties. Additionally, during tensile deformation, stabilizing austenite prolongs L & uuml;ders band strain, while insufficient stability shifts the tensile curve from L & uuml;ders band strain to continuous yielding.